This invention relates to a current source circuit having a high current mirror ratio, and more particularly to such a current source entirely composed of a translinear cross-quad circuit.
Several approaches have been used to obtain high-current-ratio current sources. One approach employs a standard basic current mirror circuit modified with feedback to augment the base current of the output transistor. Such a current mirror circuit is described in the patent to Robert Genesi, U.S. Pat. No. 4,085,411 issued Apr. 18, 1993 and assigned to the same assignee as is the present invention.
Another approach is the use of translinear circuits employing a number of bipolar devices, the PN-diode and base-emitter junctions of which are connected in a closed loop and the circuit current ratio is directly related to the product and quotient of the base-emitter junction areas. The circuit of FIG. 1 is an example of such a translinear current source.
The diodes Q1, Q2, Q3, Q4 and Q5 are seen to be in a closed loop that is completed by the base-emitter junction of transistor Q6. This leads to a current ratio ##EQU1## wherein A2,A3,A4,A5,A6 and A7 are the PN-junction areas of the respective bipolar devices Q2,Q3,Q4,Q5,Q6 and Q7. If the areas of devices Q7 and Q2 have a relative value of 1, and areas of devices Q3, Q4, Q5 and Q6 are respectively 2, 4, 4 and 8, then the current gain I.sub.out /I.sub.in is 85.3.
Current sources of the above mentioned type have a current ratio characteristic that is dependent upon the current gains of the transistors, making the manufacture of such current sources to a tight specification of current ratio difficult, and also leading to greater sensitivity of current ratio with ambient operating temperature.
It is an object of this invention to overcome the shortcomings of the current sources of the prior art and to provide a high ratio current source circuit capable of providing high output current and having a high current ratio that is relatively independent of transistor current gains.